What did they originally look like?

Passage Tombs

Passage tombs are becoming widely thought of as more than just depositories for the dead. Some passage tombs such as those in Co. Sligo at Carrowmore and Carrowkeel are over 7,000 years old and its likely they had wooden precursors.
Passage tombs were designed to be impressive, and they were built in impressive locations taking many years to complete and decorate with the fine carvings such as in Knowth and Newgrange. Some locations (such as Newgrange) continued to hold special ritual meaning to later peoples who constructed kist type burials, stone circles, mounds, pits and wooden structures around the older monuments.
The passageway leads to a chamber which usually has recessed chambers in cruciform or sometimes L shaped configurations though some are simply a widening of the passage. Some were built to have special astronomical alignments which may indicate earlier structures such as wooden and stone rows or circles.
The roof of the chamber is usually a corbelled stone structure or a sinlge large slab in smaller tombs, built to weather proof the chamber. In Newgrange water flows through special channels away from the chamber and passage, this has kept the chamber dry for over 5,000 years.
Read a piece about passage tombs by Anthony Weir here

Court Tombs

The court tomb is the oldest remaining stone built tomb type found in Ireland, they were built over four to five thousand years ago and few remain in recognisable condition.
The court tombs consist of a semi circular, rectangular or fully circular 'court' of stones usually to the front of the tomb, from which this type of tomb gets its name. The entrance leads to a set of chambers and galleries which were roofed and the whole structure was then surrounded and covered in a cairn of stone.
Read a fascinating and informative piece about court tombs in historical, geographical and soicial context written by Anthony Weir, here.

Portal Tombs (Dolmens)

The Portal Tomb (often referred to as a 'Dolmen') consists of a chamber made of large standing stones supporting a large capstone (or two), surrounded by a cairn of small stones and sometimes earth. Some portal tombs still have the door stone in place (not pictured) which sometimes blocked the full height of the tomb, and sometimes only part of it.
More information and some beautiful photographs of portal tombs can be found on this page of Anthony Weirs site.

Wedge tombs

Wedge tombs were built sometime after portal tombs (dolmens), the stones that make up wedge tombs are not known for their aesthetic appeal like those of portal, court and passage tombs and wedge tombs appear more functional than decorative. The labour required to construct these tombs however still conveys the significance of the burial site.
They are believed to have been built during the bronze age and many are aligned to a sunrise or sunset.
Read more about these intriguing tombs here.